1. Field of the Invention
The present invention relates to a shutter of a camera and particularly to a shutter which has its shutter blades driven by electromagnetic power.
2. Description of the Prior Art
In a conventional shutter in which shutter blades are driven by power of a spring, when shutter blades are set at their positions to start running, mechanical checking means are needed to check said blades at said starting positions against the driving power of the spring.
In a shutter which is driven by electromagnetic power, driving power will not work on its shutter blades until power is supplied to an electromagnetic driving source. But, unless the shutter does not have means to hold the shutter blades at their positions to start running or their positions at which the running is completed, the shutter blades may be inadvertently moved by a shock from an outside of a camera or by a shock derived from operations of a diaphragm or a mirror of a camera itself, etc., thereby resulting in an erroneous exposure.
When mechanical tie-down means are provided to prevent such erroneous operation and exposure, it is necessary to provide a mechanism to release the tie-down means before power is supplied to the electromagnetic driving source, complicating a structure thereof. Also, it is difficult in such an arrangement to adjust timings of a beginning of the electromagnetic driving and a release of the tie-down means.
When a rotating magnet type of electromagnetic drive source is used, it is possible to hold the rotating magnet at a predetermined position by providing a notch window at a fixed yoke or shielding plate and utilize the leakage flux generated thereby. However, the space occupied by an electromagnetic drive source within the camera and the capacity of the power source have limitations. Hence, the drive torque of the electromagnetic drive source is also limited.
Thus, when a permanent magnet is used, it is fully magnetized to obtain the maximum performance therefrom. However, the magnetic powers of such magnetized magnets will vary considerably. Therefore, when shutter blades in a focal plane shutter are held at their start positions, there will often be a difference between the holding power applied to the leading shutter blade and that applied to the follower shutter blades. Thus, the running speeds of the two shutter blades, when they start running will be different. This could result in a situation wherein a correct exposure time cannot be obtained.
Using a complicated electric regulating circuit to eliminate this shortcoming introduces several disadvantages. For example, it raises the manufacturing cost of the system. Furthermore, it requires that the magnetic power of the magnet for leading shutter blades and that for follower shutter blades be made equal to each other. Hence, it becomes necessary that one of the magnets may have its magnetic power reduced. Thus, it is utilized with reduced performance.